This invention relates to a system and method for determining the location of a transmitter, such as a mobile transmitting unit, and more particularly, this invention relates to a system and method of accurately determining the location of a mobile transmitting unit in a multipath environment.
In prior art location determining systems where Time of Arrival (TOA) and/or Angle of Arrival (AOA) methods are used, multipath is a significant contributor to errors. For example, one global positioning system (GPS) based location system has a reported Root Mean Square (RMS) error of 7 meters when the GPS receiver is in a suburban sidewalk or clear area. In the more congested street canyons between high-rise buildings, however, the reported Root Mean Square error is approximately an order of magnitude greater. These errors arise from the inability of the receiver to determine times of arrival (either absolutely or comparatively) of signals propagating via straight-line paths from the satellites to the receiver in the multipath environment. In many instances, the energy in a straight-line path is not even detectable.
Some mobile phone locating systems use a network of receivers to measure the time of arrival (TOA) or the angle of arrival (AOA) of the phone signal at plural receive sites and use these measurements in a multilateration or triangulation process to determine the location of the phone. These systems require that three receivers measure the signal Time of Arrival or Angle of Arrival in order to resolve ambiguities in the location estimate. The accuracy of these cell-phone overlay systems is also degraded by multipath. Errors are introduced by the inability of the receiver to detect and measure the time of arrival or angle of arrival of that portion of the signal propagating in a straight-line path from the transmitter to the receiver. In addition, those systems that employ time-difference-of-arrival (TDOA) calculations are also subject to errors introduced by errors in the time references at the plural receive sites.
Other location systems use combined Angle of Arrival and Time Difference of Arrival (or a similar technique) where the Time of Arrival and the Angle of Arrival of the signal are measured at two or more receive sites. An advantage of these systems relative to those that use only Angle of Arrival or Time Difference of Arrival is that the location of the transmitting unit may be unambiguously determined from data from two receive sites.
One example of such a location system is shown in U.S. Pat. No. 5,719,584 to Otto, assigned to the present assignee, Harris Corporation of Melbourne, Fla., the disclosure of which is hereby incorporated by reference in its entirety. In the ""584 Otto patent, a system and method determines the geolocation of a transmitter within or without a set of receiving stations. Plural receiving stations determine the Time of Arrival and Angle of arrival of a signal from a transmitting unit, such as a mobile transmitting unit. A central processing unit determines the geolocation of the radiating unit from such Times of Arrival and Angles of Arrival data. If the measured Angle of Arrival and Time of Arrival are not that of the straight-line path, errors in the location estimate will result. This system is also subject to errors in TOA measurements caused by inaccuracies in the time references at the receive sites.
Other patents, such as U.S. Pat. No. 6,184,829, assigned to True Position, Inc., concerns a method and apparatus for calibrating a wireless location system to make highly accurate Time Difference of Arrival and Frequency Difference of Arrival measurements. A first reference signal is transmitted from a reference transmitter and received at first and second receiver systems. A first error value is compared with a measured Time Difference of Arrival or Frequency Difference of Arrival value with a theoretical time difference of arrival or a frequency difference of arrival value associated with the known locations of the receiver systems and the known locations of the reference transmitter. The first error value is used to correct subsequent Time Difference of Arrival measurements associated with the mobile transmitter to be located. An in internal calibration method injects a comb signal into the first receiver system. The comb signal is used to obtain an estimate of the manner in which the transfer function varies across the bandwidth of the first receiver. This mitigates the variation of the first transfer function on the time measurements made by the first receiver.
Another prior art location system incorporates a radio frequency xe2x80x9cfingerprintingxe2x80x9d technique in an attempt to take advantage of multipath in the location determining process. In this system a database of received signal signatures or xe2x80x9cfingerprintsxe2x80x9d is generated during a calibration process for each receiver in the system. Each calibration fingerprint consists of a set of signal parameters measured by that receiver when a transmitter is transmitting from a known location or grid point. The database consists of a large set of these fingerprints with each one referenced to its corresponding grid point. In this location system, the radio frequency fingerprint of the mobile transmitting unit to be located is measured, and the location estimate is the grid point associated with a xe2x80x9cmatchingxe2x80x9d fingerprint from the database. By using this technique, it is possible under the appropriate circumstances to locate the mobile transmitter in a high multipath environment with a single receiver.
This type of location system has several drawbacks and limitations. For example, a mobile transmitting unit is presumed to be at one of the grid points, instead of intermediate of various grid points. Thus there is an inherent location estimate xe2x80x9cquantizationxe2x80x9d error related to the density of the grid points. Generation of the fingerprint database, often referred to as xe2x80x9ccalibrationxe2x80x9d, requires the costly and time-consuming process of having a transmitter travel to and transmit from each grid point. Once the calibration process is complete, changes in the local skyline caused by the erection or tearing down of a building or other structure result in changes to the fingerprints for grid points in the vicinity and, therefore, re-calibration is required.
Relocation of a receiver also requires that the calibration process be repeated for that receiver. Typically, the grid points are at street level due to the difficulty of getting access to every building or structure during the calibration process and due to the increase in cost associated with generating a three dimensional grid as opposed to a two dimensional grid. It is well known by those skilled in the art, however, that the multipath profile, or fingerprint, of a transmitter changes dramatically in an urban environment as the transmitter goes up in elevation. Thus, a transmitter carried by an individual on the 30th floor of a building would have a fingerprint that would vary dramatically from that generated by a transmitter located on or near the ground floor of the same building during calibration. The differences may be so great that the fingerprint does not match the fingerprint for the correct grid point or, perhaps even worse, the fingerprint matches the fingerprint of another grid point some distance removed.
The fingerprint of a transmitter attached to or embedded in an object such as an asset may also vary dramatically from the fingerprint of the transmitter used for calibration due to shadowing or directional blockage induced by the object. This may result in failure to find a matching fingerprint in the database or result in a match to a fingerprint for a grid point far removed from the actual location of the transmitter. The use of the fingerprinting technique is not readily applicable to mobile receivers since calibration would have to be repeated for every possible location of the mobile receiver. The expense associated with generating a database containing fingerprints for every grid point at each of the possible locations of the mobile receiver is prohibitive.
It is therefore an object of the present invention to provide a system and method for determining the location of a transmitter that reduces errors induced by multipath due to natural or manmade objects.
It is yet another object of the present invention to provide a system and method for accurately determining the location of a transmitter that does not require a line of sight signal from a transmitter to a receiver.
It is still another object of the present invention to provide a system and method for determining the location of a transmitter that minimizes the number of receivers that must be within range of the transmitter.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter that removes or reduces errors in location estimates caused by errors in receiver time references.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter that does not require calibration via transmissions from a plurality of grid points and is not subject to the location estimate xe2x80x9cquantizationxe2x80x9d error associated with such grid points.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter that is operable with a database that is not necessarily extensive and that may be readily modified or enhanced with minimal expense.
It is yet another object of the present invention to provide a system and method for accurately determining the location of a transmitter that is tolerant of changes in the environment such as the addition or deletion of buildings or structures.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter that is tolerant of changing multipath profiles induced by people or objects near or moving about the transmitter or caused by changes in transmitter elevation.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter that is useful in systems employing fixed, mobile, or both fixed and mobile receivers.
It is yet another object of the present invention to provide a system and method for determining the location of a transmitter in three dimensions.
The present invention is advantageous and provides a system and method for determining the location of a transmitter using a multilateration calculation with the inputs to multilateration calculation including, but not limited to, the estimated Time of Arrival of the transmitted signal at a natural or manmade reflecting or refracting object and the location of the object. It should be noted that a reflecting or refracting object could be a portion of a larger object. The TOA of the signal at the object is calculated as the estimated Time of Arrival at a receiver of that portion of the signal believed to have been reflected or refracted by the object minus the known (or calculated) time of propagation from a point selected as representing the location of the object to the receiver.
The point selected as representing the location of an object so used in the multilateration process is hereafter referred to as a proxy receiver site or a proxy receiver location and the object is hereafter referred to as a proxy receiver. The processor includes in the multilateration calculation such inputs as may be required in order to determine an estimated location or set of possible locations of the transmitter.
These additional inputs may, but do not necessarily, include one or more additional proxy receiver sites and the associated estimated Time(s) of Arrival at the respective additional proxy receiver site(s). The time of arrival at a proxy receiver site may be determined from the time of arrival of the signal refracted or reflected by that proxy receiver at said receiver or at any one of a second, third, fourth, or more, fixed or mobile receivers that may be included in a specific system deployment. These additional inputs may also, but do not necessarily, include one or more input(s) as determined by other means. The inputs may include, but are not limited to, Time of Arrival, Ranging Measurements (such as round-trip time of flight), Angle of Arrival, Signal Strength or other information collected by any means as known by those skilled in the art. These additional inputs may be measured by any combination of said first, second, third, fourth, or so on receivers that may be present in the specific system deployment.
In accordance with the present invention, a system and method determines the location of the transmitter and includes the transmitter to be located that transmits a signal. A fixed or mobile receiver of substantially known or determinable location receives portions of the signal from the transmitter via one or more paths that may or may not include a line of site path. The receiver measures the Time of Arrival of at least one of the paths believed not to be a straight-line path. A processor operatively connected to the receiver selects a natural or manmade object of known or determinable location as a proxy receiver. The processor calculates the location of the transmitter based on a multilateration calculation including as inputs the estimated Time of Arrival of the signal at the proxy receiver and the known (or calculated) location of the proxy receiver. The processor also includes as inputs to the multilateration calculation other information as may be necessary to determine the location of the transmitter.